docs: def scale_edges(self, multiplier):
          '''Multiply all edges in this ``Tree`` by ``multiplier``'''
          if not isinstance(multiplier,int) and not isinstance(multiplier,float):
              raise TypeError("multiplier must be an int or float")
          for node in self.traverse_preorder():
              if node.edge_length is not None:
                  node.edge_length *= multiplier